During development, cells become committed to different fates. This process is called determination. Our ultimate objective is to discover the molecular mechanisms of determination. The imaginal discs of Drosophila melanogaster offer exceptional advantages for studying this process. Imaginal disc cells do not immediately differentiate after they are determined; instead they proliferate for approximately ten cell generations. This makes it possible to analyze the transmission of the determined state in imaginal discs free from the complications of simultaneous differentiation. The proliferative phase of development can be extended by culturing imaginal discs in vivo with no loss of capacity for terminal differentiation. In addition, several classes of mutants which affect the growth and determination of imaginal discs have been described. These mutants are powerful analytical tools. I am proposing for this project period, to identify protein molecules relevant to the transmission of the determined state in wing and haltere imaginal discs. The plan of this project involves four steps: 1) To confirm preliminary results that discs growing in our recently developed in vitro culture system are normal, we will systematically compare discs cultured in vitro to those cultured in vivo; 2) We will grow haltere discs in vitro at different rates of cell division to test the hypothesis that the frequency of transdetermination is proportional to the rate of proliferation; 3) We will identify differences in the patterns of protein synthesis in wing and haltere discs by high resolution, two dimensional polyacrylamide gel electrophoresis; 4) We will evaluate which of the haltere specific proteins are relevant to determination by examining which are affected by three homeotic mutants in which the haltere disc is transformed to wing and by examining which are affected when the haltere disc transdetermines to wing in vitro.